This invention relates to an differential pressure transducer which is especially suited for measuring the differential pressure in a severe industrial environment.
A differential pressure transducer is defined as a measurement means for measurement of differential pressure.
Differential pressure transducers are widely used for example in the chemical industry, food industry, aviation industry, in water treatment plants, etc. for purposes of among others flow measurement in flowing media, the control of filters, speed measurement in aircraft or water craft. The necessity of precise measurement of small differential pressures, typically roughly 100 mbar, around a high absolute (conventionally static) pressure of for example 100 bar, has led to development of relatively expensive and complex pressure transducers, since these devices on the one hand must demonstrate their serviceability in the transfer from ambient pressure to high absolute operating pressure and on the other hand must be sensitive enough to be able to measure small differential pressures with the desired accuracy.
WO 96/41141 discloses a pressure transducer in which a measurement membrane of electrically conductive material such as steel divides a measurement chamber into two measurement spaces. The membrane can be exposed to measurement pressures which yield a differential pressure in the two measurement chambers and it is deflected by the differential pressure from its neutral position. In one measurement chamber wall opposite the membrane there is a coil. Relative movements between the coil and membrane are measured via measuring inductance.
In pressure transducers of this type the problem is that, as a result of the elasticity of the measurement membrane, which is linear only in a narrow range, for larger deflections of the measurement membrane there is no longer a linear relationship between the change of the differential pressure and the corresponding change of inductance. Furthermore, there is the danger of inelastic deformations of the membrane which adulterate the measurement results over the long term, for which reason the long term serviceability of the pressure transducer is not ensured. In addition, temperature changes and/or corrosion can significantly change the material properties of the membrane; this can lead to further measurement errors.
DE-A1-31 06 835 describes a differential pressure measurement device which has a measurement chamber which is divided into two measurement spaces by a measurement membrane. The measurement chamber is hermetically sealed against the medium to the outside by means of separation membranes and the differential pressure can be measured in it. To transfer the differential pressure to be measured from the separation membranes to the measurement membrane, the two measurement spaces between the measurement membrane and the respective separation membrane are filled with oil. The measurement membrane in its center has a semiconductor sensor with which the deflection of the measurement membrane from its neutral position is measured. To prevent mechanical overload of the semiconductor sensor there is a compensation device which is coupled via a mechanical connection to the semiconductor sensor and thus the measurement membrane in order to counteract the deflection caused by the differential pressure. In the differential pressure measurement device as claimed in this patent the problem of mechanical overloading of the measurement membrane is reduced.
However, the differential pressure measurement means as claimed in DE-A1-31 06 835, as also otherwise all other known pressure transducers, has a complicated and expensive structure; this makes it unsuited for economical production in large series. At the same time the force of gravity in all known pressure transducers with a measurement membrane represents another error source in that under the influence of gravity the measurement membrane can already be deformed as a result of its inherent weight; this results in an error in pressure measurement which is dependent on the position of the pressure transducer.
The object of this invention is to make available a differential pressure transducer which is structurally simple, which can be economically produced in large series, and which is long-lived and itself enables reliable differential pressure measurements under ambient conditions which are severe with respect to corrosion and temperature.
The differential pressure transducer according to the invention has a measurement chamber which is defined by a housing and which is divided by a membrane device into a first measurement space which can be exposed to a first measurement pressure and a second measurement space which can be exposed to a second measurement pressure. The membrane device comprises at least one measurement membrane, the membrane device being arranged and made such that a deflection force which is engendered by the first measurement pressure and the second measurement pressure causes deflection of the measurement membrane from its neutral position. The pressure transducer furthermore has a deflection sensor means for measuring deflection of the measurement membrane from its neutral position and for producing a deflection signal depending on this deflection. The pressure transducer according to the invention is characterized in that it furthermore has an electromagnet arrangement which comprises magnet means and a microengineering current-carrying coil, and an electrical switching arrangement. The electromagnet arrangement is made to produce a magnetic force which couples the measurement membrane to the housing and the electrical switching device is used to produce a coil current depending on the deflection signal such that the magnetic force acting on the measurement membrane essentially compensates for the deflection force which is caused by the differential pressure and which acts on the measurement membrane. The coil current for producing the magnetic compensation force represents a measure of the differential pressure. The current intensity of the coil current is output as the output quantity of the pressure transducer.
Microengineering is defined as the means and processes which are employed to use and/or produce (preferably in series) extremely small technical devices or equipment. In microengineering the aspect of production, especially series production, is just as important as that of the function of a microengineering device. Microengineering components (for example, the coil of the pressure transducer as in the invention) therefore often have essentially flat, two-dimensional structures. They can be produced by means of methods of planar technology, especially printing processes (for example screen printing in thick film technology, vapor deposition in thin film technology, UV lithography).
In the context of microengineering, the term xe2x80x9csmallxe2x80x9d, at least in mechanical and/or electromechanical devices, means that their main dimensions are in the centimeter or subcentimeter range.
A xe2x80x9cmagnetic forcexe2x80x9d is the force which an electrical conductor or permanent magnet experiences in a magnetic field and xe2x80x9cmagnetic meansxe2x80x9d are means of electrically conductive and/or magnetic material which can experience a magnetic force in a magnetic field.
The differential pressure transducers known to date are not suited for production with methods of planar technology. They have components with structures which are essentially three-dimensional and their production is structurally complex. In the pressure transducer as claimed in DE-A1-31 06 835 this applies especially to the compensation device which comprises an electromagnet with a coil and a permanent magnetic armature, and the two have structurally complex three-dimensional structures. Conversely, the microengineering (and thus flat) coil of the differential pressure transducer according to the invention can be produced with means and methods of microengineering. In this way the possibility of economical, large-series production of the differential pressure transducer as claimed in the invention is established. At the same time, the microengineering structure of the coil yields an outstanding possibility for miniaturization not only of the coil itself, but the entire differential pressure transducer in accordance with the invention. By miniaturizing the pressure transducer the measurement errors which are caused by gravity can be greatly reduced, since the weight of the membrane decreases with the cube of the geometric dimensions. The reduction of the influence of gravity then allows the use of mechanically softer materials for producing the measurement membrane. In this way the choice of materials which can be used to produce the measurement membrane is greatly increased so that some chemically resistant materials can be used which, to date, had not been considered for the measurement membrane of known pressure transducers, because their low mechanical strength under the influence of gravity and/or large differential pressures has/have caused overly large measurement errors.
In the differential pressure transducer as claimed in the invention, the feedback principle is used to measure the differential pressure. The pressure difference to be measured between the two measurement spaces is applied to the membrane device. By means of the differential pressure either directly (i.e. without pressure transfer) or indirectly (i.e. after amplifying or weakening the pressure) a deflection force is produced which causes deflection of the measurement membrane of the membrane device from its neutral position. A deflection sensor measures the deflection of the measurement membrane from its neutral position, produces a deflection signal which corresponds to this deflection, and supplies it to an electrical switching device. The measurement membrane is coupled by magnetic means and an electromagnetic arrangement which comprises a current-conducting coil to the housing of the pressure transducer, i.e there is a magnetic force which is acting directly or indirectly between the measurement membrane and the housing. The switching device is electrically connected to the coil of the electromagnet arrangement and produces a coil current such that the coil interacts by means of magnetic force with the magnetic means such that overall a force is exerted by the housing on the measurement membrane which essentially compensates for the deflection force acting on the measurement membrane in order to cancel the deflection of the measurement membrane from its neutral position. This is, in turn, measured by the deflection sensor (thus the feedback loop is closed). As a measure for the differential pressure the strength of the coil current which is necessary to produce the magnetic compensation force is measured. This in contrast, for example, to the differential pressure transducers of the type described in WO 96/41141 where the deflection of the measurement membrane is used as a measure for the differential pressure. Thus, in the differential pressure transducer according to the invention, large deflections of the measurement membrane are avoided. This yields the advantage that, in the pressure transducer of the invention, the measurement membrane need not meet high demands with respect to its mechanical properties in terms of to elasticity and/or linearity. In particular, it is feasible to produce the measurement membrane of the invention using those materials which are mechanically soft and/or which have a small linear elasticity range but, for example, are especially resistant to corrosion.
The differential pressure transducer according to the invention overall represents an intelligent measurement sensor in which the pressure is measured by active compensation of the deflection of a mechanical structure, by which the measurement result becomes essentially independent of the mechanical properties of the structure.
Preferably the electromagnet arrangement is made for non-contact exertion of force between the housing and the measurement membrane by means of remote magnetic action. This embodiment of the invention enables prompt control of the opposing force which compensates for the compressive force.
The coil can be attached to the housing by gluing, embedding, welding, soldering or in some other suitable way. Preferably the coil is embedded completely in one housing wall which borders the measurement chamber so that the coil top which faces the measurement membrane is essentially coplanar to the surface of this housing wall facing the measurement membrane. This embodiment of the invention is especially advantageous when the pressure transducer is being cleaned.
Advantageously the measurement membrane is located in the measurement chamber which is defined by the housing such that the housing walls ensure effective overload protection against mechanical overloads of the measurement membrane. This is done by the distance which is perpendicular to the plane of the measurement membrane between the measurement membrane and the housing walls which border the measurement chamber being chosen to be as small ask possible. When the measurement membrane is overloaded as a result of the overly large differential pressure then the measurement membrane contacts one housing wall and is supported by it, thus preventing inelastic extension of the measurement membrane.
According to one preferred embodiment of the invention the electromagnet arrangement is made as an integral part of the deflection sensor means, to measure the deflection of the measurement membrane from its neutral position an electrical signal being measured which has been produced in the coil by relative motion between the magnetic means and the coil. In this embodiment of the invention thus the electromagnet arrangement on the one hand has the function of a servo device (or an actuator) for the measurement membrane by its acting via magnetic force on the measurement membrane in order to return it into its neutral position against the compressive force. On the other hand, the electromagnet arrangement also has the function of a deflection sensor means or a sensor for measuring the deflection of the measurement membrane from its neutral potion. Thus the sensor is at the same time also an actuator. In other embodiments of the invention the functions of the servo device and the deflection sensor means however can also be assumed by separate devices. Thus for example a Hall probe can be used to measure the deflection of the measurement membrane.
Advantageously the electromagnet arrangement is made and arranged essentially symmetrical with respect to a plane of symmetry which is defined by the measurement membrane, in order to reduce the formation of the asymmetrical temperature distributions which cause errors in pressure measurement. These asymmetrical temperature distributions are often the cause of errors in pressure measurement by means of differential pressure transducers. With the symmetrical formation of the electromagnet arrangement at the same time position-dependent errors can be actively compensated, which even in the pressure transducer as claimed in the invention are still present as a result of the action of gravity on the measurement membrane when the latter is not positioned vertically.
According to one preferred embodiment of the invention the membrane device comprises simply a single membrane in the form of the measurement membrane, the membrane device being made and the measurement membrane being arranged such that one side of the measurement membrane is directly exposed to the first measurement pressure and the other side of the measurement membrane is directly exposed to the second measurement pressure. This embodiment of the invention enables simple construction of the membrane device with simply one single membrane.
According to another preferred embodiment of the invention the membrane device comprises a first pressure pick-up membrane which can be exposed to the first measurement pressure and which is coupled to transfer force to the measurement membrane. For purposes of conversion of force per unit area (i.e. amplification or reduction of the surface force) the effective (i.e. freely suspended) surface of the first pressure pick-up membrane has a surface size which differs from the effective surface of the measurement membrane. The measurement of comparatively large differential pressures is enabled by this embodiment of the invention. The pressure pick-up membrane can be connected by means of a solid connecting part to the measurement membrane in order to transfer force. The membrane device can furthermore comprise a second pressure pick-up membrane which can be exposed to the second measurement pressure and which is coupled to the measurement membrane in order to transfer force, and for purposes of conversion of force per unit area the effective surface of the second pressure pick-up membrane can have a surface size which is different from the effective surface of the measurement membrane.
Preferably the measurement membrane consists essentially of a mechanically soft material with high resistance to corrosion. Preferably the membrane material is also resistant to other chemical reactions when it comes into contact with chemically reactive media. Possible materials for producing the measurement membrane are polytetrafluorethylene (PTFE, better known under the commercial name of Teflon), ethylene tetrafluorethylene (EFTE), polyethylene cotrifluorethylene (ECTFE), perfluoraloxypolymers (PFA), polyvinylidene fluoride (PVDF), copolymers of tetrafluormethylene/hexafluorpropylene (FEP), other fluorine-containing plastics, polyethylene (PE), stainless steel, polyesters, etc.
The measurement membrane however can also be produced from a conventional membrane material and/or by means of suitable treatment of the surface it can be protected against corrosion and/or other chemical reactions.
In one preferred embodiment of the invention the magnetic means comprise means which are formed essentially from ferromagnetically soft material. With a given coil geometry, coil current, and distance between the coil and the magnetic means thus extremely large magnetic forces can be produced between the housing and the measurement membrane. To produce the magnetic means according to this embodiment of the invention suitable ferromagnetically soft materials are amorphous metals, iron-nickel alloys (for example, as are sold under the commercial name Permalloy), silicon iron, iron, ferrites, silicon-iron-aluminum and others.
Furthermore, it is preferred that the magnetic means comprise permanent magnetic means of ferromagnetically hard material. The group of materials suitable for producing magnetic means according to this embodiment of the invention comprises aluminum-nickel, aluminum-nickel-cobalt, rare earths, neodymium-iron-boron, neodymium-iron-cobalt, hard ferrite magnets and others.
In still another version of the invention the magnetic means comprise means which are produced from electrically conductive material, preferably gold or copper. The magnetic means in this version of the invention generally do not consist of magnetic material, but rather the electrically conductive material acts only in conjunction with an alternating current as an (electro)magnet. The magnetic means can be made for example in the form of a short-circuited flat coil or in the form of a plate of electrically conductive material. In this embodiment of the invention in the coil of the electromagnet arrangement an alternating current is generated which produces a magnetic field which changes over time in the area of the magnetic means. In this way a current is induced in the (electrically conductive) magnetic means so that they act as an electromagnet and experience a force as a result of the magnetic field.
In one preferred embodiment of the invention the switching device for controlling the coil currents in two coils comprises a differential oscillator. In alternative embodiments of the invention the switching device for controlling the current or currents in one or more coils can comprise one or more bridge circuits.
The following detailed description of this invention is used in conjunction with the attached drawings only as an example for better understanding of the invention and should not be interpreted as a limitation of the scope of protection of the patent claims. For one skilled in the art other advantageous embodiments and combinations of features can be easily recognized from the following description in conjunction with the attached drawings and the totality of the patent claims and however still lie within the scope of this invention.